The present invention relates to a ferritic-austenitic steel with a high content of Cr and N and a low content of Ni, primarily intended for high-pressure components in plants for the synthesis of urea. However, it is also suited for other purposes where good corrosion resistance or high strength is required.
Duplex stainless steels are steels characterized by a ferritic-austenitic structure, where the two phases have different compositions. Modern duplex stainless steels are mainly alloyed with Cr, Mo, Ni and N. The duplex structure means that Cr and Mo will be enriched in the ferrite and Ni and N in the austenite. Most of the modem duplex steels contain 22-27% Cr, 4-7% Ni, 0-4% Mo and 0.1-0.3% N. This gives the materials a duplex structure with 30-70% ferrite and the rest austenite. Other elements, such as Mn, Cu, Si and W, also occur in order to give the alloys special properties.
Duplex stainless steels are often used as alternatives to austenitic stainless steels, at a lower price, due in part to the lower Ni content in duplex stainless steels. Thus, it is often possible to find a duplex stainless steel with a corrosion resistance corresponding to the austenitic stainless steel. An example of this is the austenitic 254 SMO.RTM. (UNS S 31254), with 20% Cr, 18% Ni, 6% Mo and 0.2% N, which has a corrosion resistance in chloride-containing environments of the same level as the duplex steel SAF 2507.RTM. (UNS S 32750), with 25% Cr, 7% Ni, 4% Mo and 0.3% N.
However, some austenific steels, such as Sandvik 2RE69, which was developed specially for use in urea processes, with 25% Cr, 22% Ni and 2% Mo, has so far lacked an adequate correspondence in duplex stainless steels for use in urea processes. This problem has been solved by the present invention.
The alloying levels in duplex stainless steels are restricted at their upper levels by the structure ability. The ferritic-austenitic structure implies that the material is sensitive to embrittlement at 475.degree. C. and separation of intermetallic phase in the temperature range of 600.degree.-1000.degree. C. Separation of the intermetallic phase is enhanced primarily by high contents of Cr and Mo but it can be suppressed by the inclusion of N. The effect of N on structural stability means that higher contents of Cr may be alloyed into the material without any deterioration of the structural stability. However, the N-content is limited upwards by its solubility in the melt, which gives rise to porosity at too high percentages, and by the solid solubility in the alloy, which may cause nitride precipitation.
In order to increase the solubility of N in the melt, the Mn and Cr contents may be increased. However, Mn increases the risk for separation of intermetallic phase, so the Mn content should be restricted. Since N is a strong austenite promoter, the Ni content can be lowered considerably by an increased N content while still maintaining a ferritic-austenitic structure.
Plants for the synthesis of urea constitute an interesting application for austenitic and duplex stainless steels. Urea is produced by a synthesis of ammonia and carbon dioxide under high pressure and high temperature. The process solution in the high-pressure part is very corrosive towards carbon steels. Therefore, special steels are used to a large extent, but also titanium and zirconium are used. However, the latter are very costly in purchase and manufacture, thus restricting their use.
Austenitic stainless steels dominate today as materials used as components in the high-pressure part of the urea process. A frequently occurring steel is Sandvik 3R60R U.G., which is modified AISI 316L (UNS S 31603) containing (nominally) 18% Cr, 14% Ni and 2.7% Mo and a carefully controlled ferrite content. In the most demanding applications, steels of the type 25% Cr-22% Ni-2% Mo (UNS S 31050) are used. A requirement for the use of stainless steels is that the passivity of the steel can be maintained. Therefore, oxygen is added to the process solution in the urea synthesis. Thus, this addition is only necessary because of a material-technical point of view, while however simultaneously causing energy and yield losses, as well as a potential safety risk at too high contents. Therefore, out of a process-technical point of view, there is a desire to reduce the addition of oxygen, if possible completely eliminating it. However, in today's processes it is difficult to guarantee that the required amount of oxygen be present in the process solution. This is the case at, e.g., the boiling of the solution, which takes place in the stripper, this being the most critical heat exchanger. Some corrosion also occurs on steels of the type Cr25-Ni22-Mo2 (UNS S 31050) under certain conditions. Corrosion on AISI 316L (UNS S 31603) mainly takes place under condensing conditions. Thus, adequate passivity cannot be upheld in all parts of the process.
Inferior material quality also causes corrosion in the urea process, which results in attacks in connection to welded parts. Inhomogeneous material is another reason for corrosion. These factors show that good structural stability is a prerequisite for good corrosion endurance in the urea solution, or at other applications where good corrosion endurance is required.
In relation with the composition of steels used in urea process components, it is well known that Cr has a beneficial influence on the corrosion resistance. A number of investigations has also shown that Ni in austenitic steels is detrimental under conditions when low contents of oxygen occur in the process solution. This results in a pronounced increase of the corrosion speed with increasing content of Ni in the steel. On the other hand, ferritic steels containing low contents of Ni have a very low corrosion under these conditions. However, the ferritic steels have large limitations as a construction material because of bad structural stability, which results in problems in connection with welding and manufacturing.
Ferritic-austenitic stainless steels are very interesting in view of several aspects, primarily as a material in the urea process. The high strength of these steels can be well used in the high-pressure part, and the moderate nickel content gives this steel type a better resistance to corrosion under oxygen-free conditions. Thus, a ferritic-austenitic steel should have a high Cr content and a low Ni content in order to have a good resistance in a urea environment at oxygen free conditions.